Various imaging methods using an MRI apparatus are known. As one of the imaging methods, there is a technique (hereinafter, referred to as a water-fat separation technique) for acquiring an image, which is separated into an image in which water is visualized with high contrast (hereinafter, referred to as a water image) and an image in which fat is visualized with high contrast (hereinafter, referred to as a fat image), using a difference in the behavior of water protons and fat protons after the application of a high frequency magnetic field. As one water-fat separation technique, there is a Dixon method of performing water-fat separation by calculation of a plurality of images obtained from echo signals measured by changing the echo time (TE). According to the method of setting the TE, there are methods, such as a two-point Dixon method and a three-point Dixon method.
The Dixon method is an imaging method using a phase rotation difference between water protons and fat protons at each TE for acquiring an echo signal, and is susceptible to static magnetic field non-uniformity. For this reason, a phase rotation component due to static magnetic field non-uniformity (static magnetic field non-uniformity map) is calculated, and correction for eliminating the influence is performed using the static magnetic field non-uniformity map. Here, when calculating the static magnetic field non-uniformity map, phase unwrapping processing is performed in order to prevent water and fat from being replaced with each other.
The phase unwrapping processing is a process of expressing a spatial phase change as a value of a phase exceeding the range of −π to π (hereinafter, referred to as “causing phase wrap”) by eliminating a discontinuous jump, which occurs because the phase exceeding the range of −π to π is expressed again in the range of −π to π, so that the spatial phase change is continuous.
Such phase wrap may occur in the entire imaging. In a case in which the phase wrap occurs in the entire imaging, water and fat are totally replaced with each other. In order to reduce the replacement between water and fat in the entire imaging in the two-point Dixon method, Patent Literature 1 discloses a technique of performing true-false determination.
There is a predetermined relationship (phase=2π×frequency×time) between the phase and the frequency. Accordingly, a technique of separating water and fat from each other using a static magnetic field non-uniformity map that is expressed as the amount of frequency shift rather than the phase rotation component is also known (Patent Literature 2). Patent Literature 2 discloses that water and fat are separated from each other by performing an iterative calculation based on the least squares estimation using a magnetic field non-uniformity map (static magnetic field non-uniformity map of the frequency) and a signal model including water protons and fat protons in a three-point Dixon method or a multi-point Dixon method. Also in this method, the static magnetic field non-uniformity map calculated by the iterative calculation based on the least squares estimation may converge to a wrong local solution (this state is referred to as “causing a local convergence”). Accordingly, similar to the phase unwrapping processing, this problem is reduced by performing processing for connecting the solution of the static magnetic field non-uniformity map so as to be spatially continuous.
On the other hand, in the MRI apparatus, a method of administering a contrast agent, repeating imaging in time series, and monitoring how the contrast agent dyes has been put to practical use. The method of acquiring images continuously in this manner is known as dynamic imaging. Also in this dynamic imaging, there is a demand to separate water and fat from each other. Therefore, a method of acquiring a water image is used in which a signal from fat is separated using the Dixon method in combination with the dynamic imaging (Patent Literature 3).